OFDM is a multi-carrier communication scheme for transmitting information data using a plurality of sub-carriers. In OFDM, the respective sub-carriers differ from one another in communication channel quality (received power, transmission error rate and the like) due to the environments of the communication channels (transmission channels). Accordingly, a method is known that measures the quality of the respective communication channels on the reception side, generates feedback information for each sub-carrier based on the measurement results, and returns the feedback information to the transmission side, thereby improving the communication performance.
However, such a method suffers from a problem of reduced information data transmission efficiency due to the amount of feedback information which increases in proportion to the number of sub-carriers. Thus, there is a technique for reducing the amount of feedback information, in which a sub-carrier grouping is previously performed on the transmission side for collecting a plurality of adjacent sub-carriers into one group, and feedback information is generated for each sub-carrier group on the reception side (see, for example, Japanese Patent Laid-Open No. 2003-169036).
In the following, a description will be given of a conventional OFDM communication system which groups sub-carriers with reference to FIG. 1.
As illustrated in FIG. 1, the conventional OFDM communication system has first communication device 501 comprising first transmitter 503 and first receiver 504, and second communication device 502 comprising second transmitter 506 and second receiver 505.
First transmitter 501 has OFDM signal generator 507 for grouping sub-carriers to generate an OFDM signal which is a transmission signal, and adaptive controller 508 for instructing link adaptation which is executed for each sub-carrier by OFDM signal generator 507.
Second receiver 505 has information reproduction unit 509 for reproducing information data from a received OFDM signal and for measuring a communication channel quality, and feedback information generator 510 for generating feedback information based on the result of measuring the communication channel quality (communication channel information) supplied from information reproduction unit 509.
OFDM signal generator 507 included in first transmitter 501 groups information data STDAT on N (N is an integer equal to or larger than two) sub-carriers in units of n (n is an arbitrary divisor of N) in order beginning with the first sub-carrier to generate M (M=N/n) sub-carrier groups. OFDM signal generator 507 also executes the link adaptation specified by control information SCTRL to set a transmission parameter for each sub-carrier group, and generates transmission OFDM signal STX which is then transmitted to second communication device 502.
The link adaptation executed by OFDM signal generator 507 includes adaptive modulation control which assigns more multi-level values for symbol modulation to sub-carriers which belong to a sub-carrier group that exhibits a higher communication channel quality, or transmission power control which allocates larger transmission power to sub-carriers which belong to a sub-carrier group that exhibits a lower communication channel quality, or the like.
Information reproduction unit 509 included in second receiver 505 reproduces information data from received OFDM signal SRX and delivers the reproduced information data as SRDAT. Information reproduction unit 509 also generates communication channel information SCEO from received OFDM signal SRX for each sub-carrier group.
Feedback information generator 510 generates feedback information STFBO for previously set sub-carrier groups based on communication channel information SCEO generated from information reproduction unit 509. Second transmitter 506 generates transmission feedback signal SFBTX including feedback information STFBO, which is transmitted to first communication device 501.
First receiver 504 receives transmission feedback signal SFBTX transmitted from second transmitter 506 (received feedback signal SVBRX), reproduces feedback information STFBO from received feedback signal SFBRX (reproduced feedback information SRFBO), and supplies reproduced feedback information SRFBO to adaptive controller 508 of first transmitter 503.
Adaptive controller 508 selects link adaptation for improving the communication channel quality for each sub-carrier group based on reproduced feedback information SRFBO, and supplies OFDM signal generator 507 with control information SCTRL including the selection result.
In this way, the conventional OFDM communication system reduces the amount of feedback information supplied from the reception side to the transmission side by generating feedback information for each sub-carrier group. Also, communication performance is improved by executing link adaptation for improving communication channel quality based on reproduced feedback information SRFBO.
However, in the conventional OFDM communication system described above, a group pattern indicative of a correspondence relationship between each sub-carrier and sub-carriers included therein remains fixed in a previously set relationship at all times. Therefore, if a difference develops between the communication channel quality of a sub-carrier group and the communication channel qualities of the respective sub-carriers included therein, a problem arises in that appropriate feedback information is not generated in accordance with the communication channel qualities of the sub-carriers.
In addition, since the link adaptation determined by the adaptive controller does not either correspond to the communication channel qualities of the sub-carriers, a problem arises in that communication performance is largely degraded as compared with the case where optimal link adaptation is executed.